Cardiff Blues fear Sam Warburton could be out for three to four months.
The Blues medical team are yet to assess the suffered by the Wales captain on the British and Irish Lions tour to Australia.
But they understand from the Lions camp that the flanker, 24, will miss the start of the Blues' season.
"I haven't had an update on Sam but I'm led to believe it could be three or four months," Blues chief executive Richard Holland said on Friday.
"It's what came out of Australia and he's only been back in the country a couple of days.
"The Blues medical staff haven't had a look at him, so they'll be working with the Lions medical staff to try to get him back into fitness as soon as possible.
"We haven't got a timescale on it but given the message that was coming out of the Lions camp, he's got a period of rehabilitation and we've got an excellent medical team who will get him back ready as soon as we can.
"So it's important that he rests up and takes a bit of time out and then we'll get him back into the Blues and see where we are."
Warburton was injured in the second Test against the Wallabies and missed the decisive over Australia in Sydney that clinched a first series win for the Lions in 16 years.
In the wake of the third Test Warburton said: "There is no rush to get back in the next two months.
"Fortunately my twin brother (Ben) is now a physio with the Blues so he can work with me in the next four weeks with my rehab.
"I will just try to get back for the start of pre-season, that would be the target for me."
There was better news on loosehead prop Gethin Jenkins, who was an original Lions squad selection but with a persistent calf injury.
Jenkins has returned to the Arms Park after a year with Toulon but is expected to be available for the start of the Pro12 campaign.
"And Gethin is under monitoring with our medical staff so we're hopeful on him being OK for the start of the season," Holland added.
Warburton travelled out to Australia nursing a knee injury that saw the open-side sit out the warm-up match against the Barbarians in Hong Kong.
The previous year, his participation in Wales' tour to Australia had after injuring a shoulder against France in the 2012 Six Nations in March.
A shoulder 'stinger' suffered against Ireland had also ruled Warburton out of the second round of the 2013 Six Nations away to France.
Even without his latest injury, Warburton would have been unlikely to feature in the start of the Pro12 - which normally kicks off at the start of September - in common with other Lions players who will undergo a period of protracted rest.
But even if he makes a quicker than expected recovery, Warburton could struggle to be match-fit in time for the opening round of the Heineken Cup on the weekend of 11 October. The Blues have been drawn in a pool containing Exeter, Glasgow and Toulon.
However, Wales' captain is likely to be fit in time to lead them in the autumn internationals in November.